At a time when Brazil is expected to bump up its sugar cane output, researchers find that converting lands from tropical savannas to sugar cane plantations could alter the local climate.

Temperatures could fall about 1 degree Celsius during the peak of the growing season and rise an equivalent amount after harvest, said researchers from Arizona State University and the Carnegie Institution for Science's Department of Global Ecology. While the results suggest that more fields of sugar cane won't drastically alter the landscape, they could affect local seasonal temperatures.

"We're basically modifying the seasonal variability," said Matei Georgescu, lead author of the study and assistant professor in the School of Geographical Sciences and Urban Planning at Arizona State University. Researchers from Stanford University also contributed to the study.

In August, the peak growing period for sugar cane in Brazil's winter, temperatures in the area around sugar cane-growing regions will be cooler. After the harvest, around December and January, the local climate warms. But on average, the experiment showed no net impacts on surface temperatures. The study was published in Geophysical Research Letters last week.

"When the crop is harvested, we are modifying the seasonal evolution. This has to be taken into consideration; you have to focus on seasons," Georgescu said.

Using regional multiyear climate models, as well as a measure of the leaf area of sugar cane, the researchers found that this local change in temperatures would not occur in the region's tropical savanna -- known as the cerrado -- where an annual harvest doesn't occur.

These temperature fluctuations are due to the albedo effect, the warming caused by the color and reflective quality of a landscape. Just as a black shirt will absorb energy in the form of heat and make the wearer feel warmer, areas of land that are dark will absorb heat and raise temperatures in the immediate climate. The bright leaves of sugar cane deflect incoming solar radiation.

The lack of leaves after the harvest also reduces the rate of evaporation, allowing the sunlight to heat the ground at a greater rate.

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The scientists also found a seasonal variation in the rate of water evaporating from plants, known as evapotranspiration. With the changes in the landscape from greater sugar cane planting, there will be a 0.3 milliliter loss per day of water, on average.

The demand for sugar cane ethanol in Brazil is expected to rise this year. An increase in gasoline prices in the country will make biofuel a more attractive option. In addition, the United States continues to import the lower-carbon Brazilian ethanol to allow fuel blenders to comply with the federal renewable fuel standard. Unlike in previous years, weather conditions for sugar cane this year are looking good, so far (ClimateWire, Feb. 13).

The Brazilian sugar trade group UNICA was unable to confirm that sugar cane plantations will be expanding in neighboring savannas in the next several years.

To date, biofuel producers have yet to incorporate local effects like albedo into their sustainability metrics because of a lack of understanding on the topic, said Barbara Bramble, chairwoman of the Roundtable for Sustainable Biofuels.

"We've definitely been thinking about it," Bramble said when asked about incorporating localized climate effects like albedo into the standards.

In 2011, a paper was published in Nature Climate Change on Brazilian sugar cane plantations and the historical changes in temperature tied to the albedo effect (ClimateWire, April 18, 2011).

ABOUT THE AUTHOR(S)

Tiffany Stecker

Tiffany Stecker writes about climate change, agriculture and forestry for ClimateWire in Washington, DC and prefers red potatoes to Yukon Gold. She tweets about science, environment and music @TiffanyStecker.

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